1. Field of the Invention
This invention relates to a method and an apparatus for direct current non-consumable electrode gas shielded arc welding, which permit high-speed welding in all positions while suitably preventing magnetic blows.
2. Description of the Prior Art
Non-consumable electrode gas shielded arc welding or tungsten electrode inert gas arc welding (hereinafter referred to as "TIG welding" for brevity) is generally recommended for welding large structures but has the following accompanying drawbacks:
(1) upon raising the welding speed to a high level, there occurs incomplete fusion between the base metal and deposited molten metal due to insufficient preheating of the base metal, and
(2) direct current TIG arc is susceptible to influences of ambient magnetic field imposed by variations in magnetization and the shape of the workpiece. As a result, magnetic blows are often experienced and, especially when it is deflected toward a previously formed bead, such give rise to such problems as incomplete fusion owing to insufficient preheating of the base metal, undesirable reductions in welding speed and remelting of the bead. Particularly, in the latter case, the bead sometimes falls off during operations in a vertical upward position or in an overhead position.
In view of the above-mentioned drawbacks, the assignee of the present inventors proposed in prior application (Japanese patent application No. 133005/78, laid open to public inspection under No. 61384/78) a method which eliminates the drawbacks noted hereinabove by counteractively utilizing the magnetic blow phenomenon. According to this method:
(A) a DC electrode is located between a non-consumable electrode and a base metal and between a filler material and the base metal; and PA1 (B) current is conducted through the respective electrodes, PA1 (C) whereby directing the arc forward in a path of travel is possible.
(i) in inverse directions when the non-consumable electrode is in a preceding position, and PA2 (ii) in the same direction when the filler material is in a preceding position;
Referring to FIG. 1 which schematically illustrates the step (B)-(i) and in which the welding operation proceeds in the direction indicated by arrow W, a shielding gas cup 5 and a tungsten electrode 2 are located forward of a filler metal wire 6 which is fed in the direction of arrow Y to follow the movement of the electrode on the rear side thereof. In this figure, reference numeral 3 denotes an arc, 4 a bead and 7 a molten pool. When the tungsten electrode 2 is imparted with a direct current of forward polarity as shown, the filler metal 6 constitutes an anode (the base metal 1 constitutes a cathode). With a direct current of inverse polarity, such are located in inversed relations. As a result, the electrode 2 and filler metal 6 carry current flows of opposite directions and the electromagnetic forces produced by the magnetic fields of these currents act repellingly with each other according to Flemming's left-hand rule, so that the arc column is deflected in a direction away from the filler metal wire 6.
In an attempt to allow for high speed direct current TIG welding by employment of the above-described method and for the purpose of weaving the arc along a path of travel, the assignee of the present inventors further proposed in prior application (Japanese patent application No. 138004/78, preliminarily published under No. 61383/80) a method of conducting a pulsated current through the filler metal. Thus, direct current TIG welding substantially overcomes the above-mentioned drawbacks and has come to be applied more frequently to large structures. However, difficulties are still encountered in all-position welding, especially, in vertical position welding. Namely, referring to FIG. 2 which illustrates the welding method of FIG. 1 as applied to an operation in vertical position and which is substantially the same as FIG. 1 except for the difference in the angle formed by the front face of the molten pool and the base metal, the filler wire 6 must not be contacted with the electrode 2 and gas cup 5 and, even if not in contact with them, should not contaminate the electrode 2. Therefore, it is necessary to keep a suitable angular space .phi..sub.1 between the filler wire 6 and the electrode 2. The filler wire 6 is dipped in the molten pool 7 as illustrated but, since the surface area of the molten pool 7 as seen from the feed direction of the filler wire 6 is very small, especially in a vertical position and the freshest bead 4' immediately beneath the molten pool is still in semi-solidified or solidifying state, the filler wire 6 is apt to be accidentally hit or caught on the bead 4'. These problems sometimes compel interruption of the welding operation and, if not, leave bruises and depressions on the bead surfaces and thus can be a cause of joint defects. The just mentioned problems are not limited to operations in vertical position and also occur in a case where the filler wire is fed at a high feed rate to hinder wide applications of the above-mentioned prior inventions.
On the other hand, U.S. Pat. Nos. 3,122,629 and 3,549,856 disclose consumable electrode gas shielded arc welding processes using a couple of power sources. Further, methods for electromagnetically controlling an arc in non-consumable electrode gas shielded arc welding are disclosed in U.S. Pat. No. 3,163,743, U.K. patent application No. 2,037,639, "R & D" Kobe Steel Engineering Reports Vol. 30, No. 2 April 1980, and materials of International Conference of The Welding Institute in May 1979.